Audio reproduction system with low frequency compensation



Feb. 24, 1970 w. ERATH 3,497,621

AUDIO REPRODUCTION SYSTEM WITH LOW FREQUENCY GOMPENSATIGN Filed June 19,19a? 227/ p m ffi I m /2 w /5 /9 K /5 a INPUT v "'v "Wv INPUT INVEN TORFAZQUE'NCY (was) United States Patent 3,497,621 AUDIO REPRODUCTIONSYSTEM WITH LOW FREQUENCY COMPENSATION Louis W. Erath, P.O. Box 22042,Houston, Tex. 77002 Filed June 19, 1967, Ser. No. 647,149 Int. Cl. H04m1/24 U.S. Cl. 179-1 8 Claims ABSTRACT OF THE DISCLOSURE A soundreproduction system including compensation means for improving the lowfrequency response of the system. The compensation means in the form ofa voltage divider having at least two serially connected parts, one partbeing connected to receive an uncompensated signal. The other partincludes a parallel resonant circuit. A compensated signal is derivedfrom the common terminal of the serially connected parts. A lightsensitive resistor in the parallel resonant circuit prevents the speakerfrom being overdriven by loud, low frequency notes. Alternatively, theparallel resonant circuit forms part of a compensated negative feedbackloop for an audio amplifier. The degenerative feedback decreases atlower frequencies, thus increasing the gain of the amplifier. The lightsensitive resistor in the circuit increases the feedback and decreasesthe amplifier gain in response to large, low frequency signals.

Background of the invention This invention relates to audio reproductionsystems and, more specifically, to means for compensating for the lowfrequency response of a speaker in such a system.

The quality of an audio reproduction system depends on how well thefrequency response characteristics of the system correspond to thefrequency response characteristics of the human ear, which extends fromabout 16 cycles per second to 20,000 cycles per second. However, thefrequency response of most speakers used in such systems falls offsubstantially at lower frequencies. For example, the response may dropon the order of 30 decibels from 100 c.p.s. to 30 c.p.s.

Circuits have been designed to compensate for the drop in frequencyresponse of a speaker at the lower frequencies. The main drawback ofthese circuits, though,

is that the speaker is overdriven by loud, low frequency notes, whichresults in the speaker producing undesirable, harsh noise.

Accordingly, an object of this invention is an improved soundreproduction system.

Another object of the invention is means for improving the low frequencyresponse of the speaker in an audio reproduction system.

Still another object of the invention is means operable with an audioamplifier for providing to a speaker a signal compensated in response toboth the amplitude and frequency of an audio signal.

Summary of the invention Briefly, the low frequency compensation meansof this invention includes a voltage divider circuit having at leastfirst and second serially connected parts. The first part is connectedto receive an uncompensated audio signal, and the compensated signal istaken from the common terminal between the first and second parts. Thesecond part comprises a low frequency parallel resonant circuit whichincludes a light sensitive resistor, the re- "ice sistance of whichvaries inversely with the brightness of an impinging light. A lightsource is optically coupled with the light sensitive resistance, thebrightness of the light source varying with the amplitude of theuncompens'ated signal. Alternatively, the light source may be connectedin the circuit so that the brightness varies with the amplitude of thecompensated signal. A high frequency filter may be provided to bypasshigher frequency voltages around the light source. The voltage dividercircuit may be suitably connected in the audio reproduction system toprovide a compensated signal to the audio amplifier or a compensatedsignal to the speaker. Also, the parallel resonant circuit may form partof a compensated negative feedback loop for the audio amplifier. Thedegenerative feedback decreases at lower frequencies, thus increasingthe gain of the amplifier. The light sensitive resistor in the circuitincreases the feedback and decreases the amplifier gain in response tolarge, low frequency signals.

The invention will be more readily understood from the followingdescription and appended claims when read in conjunction with thedrawing.

Brief description of the drawing FIGURE 1 is a schematic diagram of oneembodiment of the invention wherein the compensated signal taken fromthe compensation means is applied directly to the speaker;

FIGURE 2 is a schematic diagram of an embodiment of the inventionwherein the compensated signal taken from the compensation means isapplied directly to the audio amplifier;

FIGURE 3 is a schematic diagram of another embodiment of the inventionwherein the compensation signal taken from the compensation means isapplied directly to the audio amplifier;

FIGURE 4 is a schematic diagram of still another embodiment of theinvention wherein compensated negative feedback is provided for theaudio amplifier, and

FIGURE 5 is the frequency response of a speaker and a family ofimpedance curves plotted against the same frequency axis.

Like elements have the same reference numerals in FIGURES 1-4.

Description of the preferred embodiments Referring now to FIGURE 1,audio amplifier 10 is connected through input terminals 12 and 13 to asource of audio signals (not shown) such as, for example, a radio tuner,phonograph transducer cartridge or the like. The output of the audioamplifier is connected to resistor 14 which forms a first part of avoltage divider circuit. A second part of the voltage divider circuitincludes resistor 15 and parallel resonant circuit 16 which are seriallyconnected between resistor 14 and ground, The parallel resonant circuitincludes a capacitor 17, resistor 18, in ductor 19, and a lightsensitive resistor 20, such as a cadmium sulfide photocell for example.Also connected to the output of the audio amplifier is a resistor 23which is serially connected through a light source 24-capacitor 25parallel combination to ground. Resistor 23 and capacitor 25 function asa high frequency bypass filter. Light source 24 is optically coupled tolight sensitive resistor 20, the two elements preferably mounted withinthe same housing. The input to speaker 11 is connected to the commonterminal of resistors 14 and 15.

The compensation means comprising the described voltage divider circuitis designed to compensate for the drop in frequency response of thespeaker at low frequencies, especially below c.p.s. The input signal, Sto the speaker is related to the output signal, S of the audio amplifieras follows:

where R14 is the resistance of resistor 14 R15 is the resistance ofresistor 15 Z16 is the impedance of parallel resonant circuit 16 ample,the value of Z16 is a maximum, and

SO and at higher frequencies, above 100 c.p.s. for example,

Rl H-Rlf) It is seen that S; increases as a function of S at lowfrequencies thus compensating for the drop-off in frequency response ofthe speaker.

Light source 24 and light sensitive resistor 20 in the parallel resonantcircuit cooperatively function to prevent the speaker from beingoverdriven in response to loud, low frequency notes from the audiosource. When a large, low frequency signal is received by the audioamplifier, the increased signal output of the amplifier causes lightsource 24 to emit more light, which in turn decreases the resistance oflight sensitive resistor 20 and the impedance of the parallel resonantcircuit. The reduced impedance of the parallel resonant circuit, Z16,may then be negligible in comparison to the resistance R14 of resistor14. In this case the value of the input signal to the speaker, S willremain about one-eleventh 5 of the output signal of the amplifier, S

FIGURE 5 illustrates the frequency relationship between the frequencyresponse characteristics of the speaker and the impedance of theparallel resonant circuit, the curve 41 representing the responsecharacteristics of the speaker and the family of curves 42 representingthe impedance of the parallel resonant circuit at various levels oflight intensity of the light source 24. The speaker response curve 41tends to be flat above 100 c.p.s., but drops off below 100 c.p.s. It isseen that the maximum impedance of the resonant circuit occurs at theresonant frequency, c.p.s., and ranges from a large value when little orno light impinges on the light sensitive resistor to a negligibly smallvalue as the impinging light increases.

The embodiment illustrated in FIGURE 2 provides the compensated signalto the input of audio amplifier 10. In this embodiment, theuncompensated input signal from the audio source (not shown) isconnected to resistor 14 of the voltage divider circuit which comprisesserially connected resistor 14, resistor 15 and parallel resonantcircuit 16. Light source 24 shunted by capacitor 25, is seriallyconnected with resistor 23 between the input terminal of resistor 14 andground. Again, the light source is optically coupled to the lightsensitive resistor 20 in the parallel resonant circuit. The compensatedsignal is taken from the common terminal of resistor 14 and resistor 15,and the signal is applied to input terminal 12 of audio amplifier 10.The output of the audio amplifier is connected directly to speaker 11.Operation of the circuit is similar to the operation described above forthe circuit of FIGURE 1.

FIGURE 3 is another embodiment of the invention wherein the voltagedivider circuit provides a compensated signal input to the audioamplifier 10, but the light source 24, optically coupled to lightsensitive resistor 20, is connected to the output of the audioamplifier. In this embodiment, energy feedback is employed by theoptically coupled light source and light sensitive resistor, whereas theembodiments in FIGURE 1 and FIGURE 2 do not utilize any feedback. It isalso to be noted in FIGURE 3 that a high frequency bypass filter is notutilized in conjunction with light source 24, nor is a fixed resistorincluded in the parallel resonant circuit 16. The inclusion of theseelements in the circuit is not essential, but depends upon particulardesign considerations.

In FIGURE 4, the compensated signal is again taken from the commonterminal of resistors 14 and 15 and applied to the input to amplifier10, but the parallel resonant circuit 16 forms part of a negative ordegenerative feedback loop for the amplifier. Negative feedback willstabilize the frequency response of an amplifier but at the expense ofamplifier gain. The impedance of the feedback loop in FIGURE 5 increasesat lower frequencies due to the resonance of circuit 19, thus thedegenerative feedback is reduced and the amplifier gain increases.However, the cooperative action of the light source 24 andphotosensitive resistor 20 decreases the feedback loop impedance inresponse to large, low frequency signals. Thus, the degenerativefeedback is increased and the amplifier gain is reduced. Consequently,the speaker is not overdriven.

While the invention has been described with reference to preferredembodiments, this description is not to be construed in a limiting sensebut for purpose of illustration only. Various modifications andadaptions may occur to those skilled in the art without departing fromthe spirit and scope of the invention as defined by the ap pendedclaims.

What is claimed is:

1. A low frequency signal compensation system for use with an audioamplifier and speaker in audio reproduction system, comprising:

a voltage divider circuit having at least first and second seriallyconnected parts, the compensated signal being taken from the commonterminal thereof,

means connecting said first part to receive the uncompensated signal,

said second part comprising a low frequency parallel resonant circuitincluding a light sensitive resistor the resistance of which varyinginversely with the brightness of impinging light, and

a light source optically coupled with said light sensitive resistor, thebrightness of said light source varying with the amplitude of theuncompensated signal.

2. A low frequency compensation system in accordance with claim 1wherein said first part is a fixed resistor, said means connects saidresistor to receive the uncompensated signal from the output of saidaudio amplifier, and second means connects the input to said speaker tosaid common terminal of said first and second parts of said voltagedivider circuit.

3. A low frequency signal compensation system in accordance with claim 1wherein said first part of said voltage divider circuit is a fixedresistor, said means connects said resistor to a source of audiosignals, and second means connects the input to said audio amplifier tosaid common terminal of said first and second parts of said voltagedivider circuit.

4. A low frequency signal compensation system for use with an audioamplifier and speaker of an audio reproduction system, comprising:

a voltage divider circuit having at least first and second seriallyconnected parts, the compensated signal being taken from the commonterminal thereof,

said first part comprising a fixed resistor connected to receive signalsfrom a source of audio signals,

said second part comprising a low frequency parallel resonant circuitincluding a light sensitive resistor varying inversely with thebrightness of impinging light,

means connecting said common terminal to the input of said audioamplifier, and

a light source connected to the output of said audio amplifier andoptically coupled with said light sensitive resistor, the brightness ofsaid light source varying with the amplitude of the output signal fromsaid audio amplifier.

5. A low frequency compensated sound reproduction system comprising:

a source of audio signals,

an audio amplifier,

means connecting said source of audio signals to the input of saidamplifier,

an audio speaker,

means connecting the output of said amplifier to said audio speaker,

feedback means connected between the output and the input of saidamplifier,

said feedback means comprising a low frequency parallel resonant circuitincluding a light sensitive resistor the resistance of which varyinginversely with the brightness of impinging light, and

a light source optically coupled with said light sensitive resistor, thebrightness of said light source varying with the amplitude of the outputsignal from said audio amplifier.

6. The low frequency compensated sound reproduction system defined byclaim 5 including a high frequency pass filter connected in parallelwith said light source.

7. An audio amplifier circuit comprising:

an audio amplifier having an input terminal and an output terminal,

a negative feedback loop connected between said input and outputterminals,

said negative feedback loop comprising a low frequency parallel resonantcircuit including a light sensitive resistor the resistance of whichvarying inversely with the brightness of impinging light, and

a light source optically coupled with said light sensitive resistor, thebrightness of said light source varying with the amplitude of the outputsignal of said audio amplifier. 8. A sound reproduction systemcomplementary with the normal response characteristics of the human earcomprising:

a source of audio signals, amplifier means for amplifying said audiosignals, low frequency compensation means operably connected with saidamplifier means, said low frequency compensation means comprising avoltage divider circuit including first and second serially connectedparts,

said first part including a fixed resistor connected to the output ofsaid amplifier means,

said second part including a low frequency parallel resonant circuithaving a light sensitive resistor therein, the resistance of said lightsensitive resistor varying inversely with the brightness of impinginglight,

a light source optically coupled with said light sensitive resistor, thebrightness of said light source varying with the amplitude of the outputsignal from said audio amplifier, and

a speaker connected to receive low frequency compensated signals fromsaid common terminal of said first and second parts of said lowfrequency compensation means.

References Cited UNITED STATES PATENTS KATHLEEN H. CLAFFY, PrimaryExaminer

